Deuteronomy 19:5-7

New Revised Standard Version, Anglicised Catholic Edition

⁵ Suppose someone goes into the forest with another to cut wood, and when one of them swings the axe to cut down a tree, the head slips from the handle and strikes the other person who then dies; the killer may flee to one of these cities and live. ⁶ But if the distance is too great, the avenger of blood in hot anger might pursue and overtake and put the killer to death, although a death sentence was not deserved, since the two had not been at enmity before. ⁷ Therefore I command you: You shall set apart three cities.